The present invention relates to an optical isolator and more specifically to an optical isolator of a two-stage type for optical communications system. The present invention provides an optical isolator in which Faraday rotators and polarizers, that are the optical parts, are arranged in an inclined configuration relative to an optical axis, and optical axes of the incident light and the exit light are brought into alignment with each other in order to remove those light beams reflected from the optical parts and, hence, to obtain high degrees of isolation without causing the optical axis to be deviated.
The optical isolator is an irreversible optical device having a function that permits the transmission of a light beam in one direction but blocks the transmission of the light beam in the opposite direction. The optical isolator is used, for example, for an optical communication system that uses semiconductor laser as a source of light beam in order to prevent the laser beam from returning to the side of the source of the light beam by reflection.
The optical isolator constitutes an isolator function unit by combining the Faraday rotator and the permanent magnet, and arranging the polarizers on both sides thereof. To enhance isolation performance, such an isolator function unit may often be arranged in two stages in series in the direction of the optical axis.
As for the prior art related to the isolator of a one-stage type, there has been proposed an optical isolator in which the incidence/exit surfaces of the polarizers and the Faraday rotator have a predetermined angle with respect to the incident and exit optical axes which are on a straight line, in order to prevent the light-beam from being reflected and returned by the incidence/exit surfaces, as disclosed in Japanese Patent Publication (Unexamined) No. 62-118315, published May 29, 1987.
The optical isolator of the two-stage type exhibits very strong isolation (greater than 60 dB) compared with that of the one-stage type. With the conventional general two-stage optical isolator, however, the light beam reflected by the surfaces of optical parts such as polarizers and Faraday rotators returns to the laser side although the amount of reflection is small. Therefore, even though the light returning to the laser side can be successfully prevented by the optical isolator maintaining isolation which is as great as 60 dB or higher, it is not allowed to prevent the light beam reflected and returned by the isolator itself, or by surfaces of the optical parts, consequently offsetting the meaning and effect of high isolation.
In the optical isolator disclosed in the above-described Japanese Patent Publication No. 62-118315 in which the incidence/exit planes of optical parts have predetermined angles with respect to the incident/exit optical axis, the isolation characteristic is not always satisfactory since it is constituted in one stage.
In an anticipatory construction by employing simply two isolator portions connected in series, the total length of the optical isolator becomes unfavorably long. Furthermore, an inclined bore must be formed in the permanent magnet to mount the Faraday rotator, hindering the workability and, moreover, making it difficult to mount the Faraday rotator maintaining high precision.